In rechargeable electrochemical cells, weight and portability are important considerations. It is also advantageous for rechargeable cells to have long operating lives without the necessity of periodic maintenance. Rechargeable cells may be used as direct replacements for primary AA, C, and D cells in numerous consumer devices such as calculators, portable radios, and flashlights. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable electrochemical cells can also be configured as larger cells that can be used, for example, in industrial, aerospace and electric vehicle applications.
The best rechargeable cell is one that can operate as an "install and forget" power source. With the exception of periodic charging, a rechargeable cell should perform without attention and should not become a limiting factor in the life of the device it powers.
There are two basic types of nickel metal hydride rechargeable hydrogen storage materials ("Ni-MH materials")the AB.sub.2 type and the AB.sub.5 type. These types of material are discussed in detail in U.S. Pat. No. 5,096,667 to Fetcenko, et. al, the contents of which are incorporated herein by reference. The term "Ovonic alloy" is frequently used to refer to all AB.sub.2 type materials in deference to their development from amorphous thin film materials discovered by Stanford R. Ovshinsky. Ovonic alloys are described in U.S. Pat. No. 4,551,400 for Hydrogen Storage Materials and Methods of Sizing and Preparing the Same for Electrochemical Applications (hereinafter the '400 patent) to Sapru, Hong, Fetcenko and Venkatesan, the contents of which are incorporated herein by reference.
As used herein, the term "Ovonic Base Alloy" refers to an AB.sub.2 alloy having a base alloy or grain phase (as this term is described in the '400 patent) containing 0.1 to 60 atomic percent Ti, 0.1 to 25 atomic percent Zr, 0. 1 to 60 atomic percent V, 0. 1 to 57 atomic percent Ni, and 0.1 t 56 atomic percent Cr.
In general, Ni-MH hydrogen storage cells or batteries (referred to collectively as "Ni-MH cells") utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in an alkaline electrolyte.
Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion: ##STR1##
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron.
The reactions that take place at the positive electrode of a secondary cell are also reversible. For example, the reactions at a nickel hydroxide positive electrode in a Ni-MH cell are: ##STR2##
A suitable separator is usually positioned between the electrodes of Ni-MH cells. The electrolyte is generally an alkaline electrolyte, for example, 20 to 45 weight percent potassium hydroxide. Lithium hydroxide may also be present in limited quantity.
A Ni-MH cell has an important advantage over conventional rechargeable cells and batteries: Ni-MH cells have significantly higher specific charge capacities (both in terms of ampere hours per unit mass and ampere hours per unit volume) than do cells with lead or cadmium negative electrodes. As a result, a higher energy density (in terms of watt hours per unit mass or watt hours per unit volume)is possible with Ni-MH cells than with conventional systems, making Ni-MH cells particularly suitable for many commercial applications.
Rechargeable cells are generally either vented cells or sealed cells. During normal operation, a vented cell typically permits venting of gas to relieve excess pressure as part of the normal operating behavior. In contrast, a sealed cell generally does not permit venting on a regular basis. As a result of this difference, the vent assemblies and the amounts of electrolyte in the cell container relative to the electrode geometry both differ significantly.
Vented cells operate in a "flooded condition." The term "flooded condition" means that the electrodes are completely immersed in, covered by, and wetted by the electrolyte. Thus, such cells are sometimes referred to as "flooded cells." A vented cell is typically designed for normal operating pressures of about 25 pounds per square inch after which excess pressures are relieved by a vent mechanism.
A variation of the vented, cylindrical, rechargeable cells of the prior art are the "one time only" venting cells where, for example, a rupturable diaphragm and blade apparatus is employed. As internal cell pressure increases, the blade is forced against the diaphragm. As the pressure increases further, the blade punctures the diaphragm, allowing excess gases to escape. This destructive type of venting mechanism is both unpredictable from batch to batch and from cell to cell within a batch. Moreover, destructive venting is good for only one excessive pressure situation. After the diaphragm is punctured, it cannot even sustain normal cell operating pressures.
In contrast, sealed cells are designed to operate in a "starved" electrolyte configuration, that is with a minimum amount of electrolyte. The enclosure for a sealed cell is normally metallic and designed for operation of up to about 100 p.s.i. absolute or higher. Because they are sealed, such cells do not require periodic maintenance.
Typically, a sealed rechargeable cell uses a cylindrical nickel-plated steel case as the negative terminal and the cell cover as the positive terminal. An insulator separates the positive cover from the negative cell can. The electrodes are wound to form a compact "jelly roll" with the electrodes of opposite polarity isolated from each other by a porous, woven or non-woven separator of nylon or polypropylene, for example. A tab extends from each electrode to create a single current path through which current is distributed to the entire electrode area during charging and discharging. The tab on each electrode is electrically connected to its respective terminal.
In sealed cells, the discharge capacity of a nickel based positive electrode is limited by the amount of electrolyte, the amount of active material, and the charging efficiencies. The charge capacity of a Ni-MH negative electrode is limited by the amount of active material used, since its charge efficiency is nearly 100 percent, nearly a full state of charge is reached. To maintain the optimum capacity for a Ni-MH electrode, precautions must be taken to avoid oxygen recombination or hydrogen evolution before full charge is reached. This is generally accomplished by providing an excess of negative electrode material. However, precautions must be taken in the design and fabrication of sealed cells to avoid the effects of over-pressurization associated with overcharge at dangerously high charge rates. Sealed cells are the preferred type of rechargeable, Ni-MH electrochemical cells where a particular application requires a relatively maintenance-free power source.
The operational life span, that is, the available number of charge and discharge cycles of a sealed cell, typically determines the kinds of applications for which a cell will be useful. Cells that are capable of undergoing more cycles have more potential applications. Thus, longer life span cells are more desirable.
The life span of a sealed cell is directly related to the life span of its individual components. The negative electrode materials are the most unique and have long been considered to be the component limiting cell life span. Therefore, to achieve longer life spans, researchers concentrated their efforts on producing an electrode alloy material capable of withstanding repeated charge and discharge cycles without breakdown. See, for example, U.S. Pat. No. 4,728,586 for Enhanced Charge Retention Electrochemical Hydrogen Storage Alloys and an Enhanced Charge Retention Electrochemical Cell, the disclosure of which is incorporated by reference. However, Ovonic alloy negative electrode materials have been developed to the point where they are no longer the only component of the rechargeable cell that limit the life span of the entire cell.
The present inventors have found that in sealed Ni-MH cells using some of the Ovonic Base Alloys described herein, cell failure is sometimes the result of problems related to the separator, such as the depletion of electrolyte from the separator and separator degradation. Thus, given the advances in Ovonic Base Alloy negative electrode materials described herein, the separator appears to be another factor in the cycle life and charge retention of the Ovonic Base Alloy cells of the present invention.
Nylon separators have been described in a variety of configurations and in a variety of cells. For example, U.S. Pat. No. 3,147,150, to Carl et al. of Yardney, describes a battery using a separator made from a nylon film coated on fabric or resinous fiber. In addition, U.S. Pat. No. 4,699,858, to Masaki of Freudenberg, describes a rechargeable alkaline battery having an ion transporting alkaline electrolyte and a nonwoven polyamide separator made from continuous fiber of 3 to 10 microns treated with non-ionic surface active agents.
Generally, nylon separators of the type found in NiCd rechargeable batteries have been used as the standard separator in all types of Ni-MH cells. Such nylon separators were specifically designed to prevent the short circuits that occur in NiCd cells due to the formation of dendrites between the electrodes as they change from a metal to a metal hydroxide in the charge/discharge cycle. These dendrites short circuit the NiCd negative terminal to the positive terminal if left unchecked. The nylon separator acts as a barrier layer to prevent such dendrite formation.
Although dendrite formation is not a concern in Ni-MH cells, nylon separators are frequently used in Ni-MH cells because during the early development of Ni-MH cells, nylon separators were readily available and appeared to function adequately with AB.sub.5 alloy cells, as well as with Ovonic alloy cells, of the prior art. However, the present invention recognizes that in the Ovonic Base Alloy cells of the present invention, prior art nylon separators do not function adequately. The prior art nylon separators when used in Ovonic alloy cells are, for example, prone to breakdown and loss of electrolyte (drying out) after repeated cycling. Further, prior art nylon separators eventually react with the electrolyte in Ovonic alloy cells forming decomposition products which may adversely effect cell performance. As a result, nylon separators when used in Ovonic alloy cells appear to be a significant factor in limiting the potential cycle life of such cells.
Polypropylene separator materials have been used in lead acid batteries because of their resistance to sulfuric acid. For example, U.S. Pat. No. 3,870,567, to Palmer et al. of W. R. Grace, describes separators made from nonwoven mats that are compressed to yield small pores of high porosity. These mats are formed from hydrophobic polymeric materials. The fibers of the mat are made wettable by mixing the polymeric resin with a wetting agent prior to extrusion. This reference specifically teaches that polyolefins such as polypropylene are useful in lead acid batteries, and that nylon is the preferred material for use in alkaline batteries.
The following art suggests polypropylene separators as an alternative to nylon because polypropylene is strong and resistant to alkaline electrolyte, although its hydrophobic properties when compared to nylon require treatment with a wetting agent or its combination with another material:
U.S. Pat. No. 3,907,604, to Prentice of Exxon Research, describes a nonwoven polypropylene mat that has been fuse-bonded using a press to increase its tensile strength.
U.S. Pat. No. 3,947,537 to Butin et al. of Exxon Research, describes a process for making battery separators from nonwoven mats where the formed nonwoven mat is treated with a wetting agent, dried, heated, and compressed to increase fiber-to-fiber bonding.
U.S. Pat. No. 4,190,707 to Doi et al. of Asahi, describes a separator made of a porous polyolefin film having low electrical resistance and high alkaline resistance.
U.S. Pat. No. 4,414,090, to D'Agostino of RAI Research, describes a separator for a redox cell comprising a polyolefin base film grafted to a vinyl substituted monomer with gamma radiation.
U.S. Pat. No. 4,430,398, to Kujas of RCA, describes a polypropylene separator for NiCd cells prepared from knitted, woven, or nonwoven polypropylene that is treated with a corona discharge and then impregnated with phenylglycine or parahydroxyphenylglycine. The corona discharge functions to increase the wettability of the separator, and glycine derivative acts to prevent the penetration of the separator sheet by the alkaline electrolyte and metallic particles from the electrodes.
U.S. Pat. No. 5,077,149, to Ikoma of Matsushita, describes a misch metal negative electrode, a nickel hydroxide positive electrode, and a sulfonated, non-woven polypropylene separator. The negative electrode, the positive electrode, and the separator all contain a zinc compound, such as zinc oxide, so that the electrolyte is retained in the negative electrode and the separator and does not migrate to the positive electrode, thus reducing the expansion of the positive electrode. In addition, the separator is treated with a hydrophobic resin. This patent states that expansion of the positive electrode causes a change in the electrolyte distribution and an increase in internal resistance which makes nickel/hydrogen cells have an inferior cycle life compared to NiCd cells.
The plethora of references describing different kinds of separators contains no indication that any one type of separator would be superior for any particular application or superior with any particular alloy or electrolyte. While all types of batteries have similar component parts, the extreme differences in chemistry make applying teachings from one type of battery to another unpredictable. "Swapping" of components does occur, but the present inventors are unaware of any instance where any research in the development of Ovonic alloys has been hastened as a result. For example, the use of NiCd nylon separators in Ni-MH cells as discussed above, proved adequate for all Ni-MH alloys initially, however these same separators now appear to be a limiting factor in realizing the full potential of the Ovonic Base Alloys of the present invention. Unfortunately, the prior art contains no theory or suggestion regarding a separator that might overcome this problem.
The inadequacy of the prior art is illustrated in U.S. Pat. No. 5,077,149 ("the '149 patent"), to Ikoma of Matsushita, discussed above. Read in its entirety, the '149 patent focuses on controlling the swelling of the positive electrode with zinc and fails to disclose any teaching relevant to the Ovonic Base Alloys of the present invention. The '149 patent teaches using misch metal, an AB.sub.5 Ni-MH material that those of skill in the art know has an electrochemical behavior different than Ovonic alloys. A number of manufacturers became interested in AB.sub.5 alloys because these alloys appeared to be drop in replacements for NiCd negative electrodes. However, as discussed in detail in U.S. Pat. No. 5,096,667, AB.sub.5 materials represent a different class of materials from Ovonic alloys. This is particularly true of the Ovonic Base Alloys of the present invention.
It should be noted that prior art Ovonic alloys have yielded adequate performance when used as a drop in replacement negative electrode for NiCd cells. However, the performance of Ovonic Base Alloys of the present invention can be significantly improved by using an optimized separator material.
In general, the references discussed above, contain no teaching or suggestion that some Ovonic Base Alloys cells will have an improved cycle life and reduced self-discharge when used with an appropriately chosen separator.